The use of motorized exercise or rehabilitation equipment to generate resistive loads for a user and obviate the need for weights are well known. While some motorized resistive systems can be operated to vary the resistive load during certain portions of an exercise cycle and thereby effectively provide an equivalent of assistance, there are some experts who believe that the use of actual weights in training or rehabilitation, with assistance for portions of the exercise, achieves a superior result.
Apparatus to generate assistive loads for a user moving a primary load of weight(s) for exercise or rehabilitation are much less common due to the more numerous and different problems encountered from mounting to control when compared to resistive force systems. U.S. Pat. No. 4,765,611 describes an early hydro-mechanical assistive system that employs counter weights to reduce the primary weight load sustained by a user. All known motorized assistive force apparatus have employed similar counter weight stacks, mounted in their own frames, making such devices quite bulky and heavy. These devise operate by supporting an counter weight stack until assistance is needed and then suddenly removing the support of all or a portion of the stack by a motor and then returning the support to the entire stack at the appropriate time in the exercise cycle. Such systems use common motors that are operated at full torque output when powered and typically controlled for “bang-bang” on/off operation by the use of position switches or proximity detectors.